1. Field of the Invention
The present invention relates to a light detector for ultraviolet radiation, and more particularly to a light detector having a frequency converting coating for converting the ultraviolet radiation to visible light before the light impinges upon a light sensitive element of the detector.
2. Discussion of the Related Art
The highest achievable resolution of a structure on silicon using light as a photolithographic tool is generally on the order of the emission wavelength of the (excimer laser) light source. The formation of structures on silicon substrates in the range of 100 nm thus uses photomicrolithography techniques employing deep ultraviolet (DUV, or 350-190 nm) or vacuum ultraviolet (VUV, or 190-100 nm) light sources as exposure tools.
Special techniques have been developed that enable the formation of structures as small as about half of the emission wavelength of the photolithographic light source used. Such techniques utilize narrowed excimer laser output beams. For example, when refractive optics are employed, the output beam can be narrowed to have an emission bandwidth of less than 0.6 pm. The wavelength stability is typically less than .+-.0.15 pm. When reflective or catadioptric optics are employed, the output beam is narrowed to have an emission bandwidth in a range from 15 pm to 100 pm. The wavelength stability of the output beam in this case is less than .+-.5 pm.
Perhaps the most efficient light source for producing 100 nm structures is the Argon-Fluoride (ArF)-excimer laser which emits light in a band centered around 193.3 nm and has a natural emission bandwidth around 500 pm. To form structures on silicon substrates having widths less than 100 nm, even smaller wavelength radiation sources are needed. The F.sub.2 -excimer laser emitting around 157 nm is a possible light source to be used for this purpose. Recent advances have already enabled the formation of 80 nm structures.
The high wavelength stabilities described above that help to enable photolithographic formation of structures half the size of the radiation wavelength are possible because several parameters associated with the laser output beam are monitored. Some of these parameters include the emission wavelength, bandwidth, pulse energy, and beam profile. The wavelength monitoring may be performed using a spectrometer such as one including a wavelength selection unit and a detector.
A problem exists with conventional detectors when they are exposed to short wavelength radiation, e.g., having a wavelength around 193 nm, 157 nm, or 209-219 nm, such as that emitted from an ArF-excimer laser, an F.sub.2 -laser, or a frequency-quintupled Nd-laser (Nd-YAG, Nd-YLF, Nd-YAP, Nd-YLJ e.g.), respectively. When energy densities in the range of nanojoules/cm.sup.2 are directly incident on a conventional detector at these short wavelengths, a dark current background is generated which rapidly increases in proportion to a total accumulated exposure dose.
The dark current signal is background noise that superimposes itself over desired detection signals, and acts as an undesirable background artifact of spectra generated from data measured by the detector. Dark currents are also generated when detectors are exposed to longer wavelengths. However, the dark current background generated when the same detectors are exposed to KrF-excimer laser source radiation around 248 nm, increases with total accumulated exposure dose at least an order of magnitude slower than when exposed to 193 nm radiation. It is believed that the rapid increase in the dark current background with total accumulated exposure dose observed at shorter wavelengths is caused by changes induced by high energy photons in semiconductor chips used to fabricate the detectors.
The end result is that a conventional detector exposed to, e.g., an ArF-excimer laser, a F.sub.2 -laser, radiation will generate a greater and greater dark current background signal with exposure time, until the detector becomes useless. That is, after a period of time, or the "lifetime" of the conventional detector, the dark current background becomes so intense that it becomes undesirable to monitor any of the laser parameters mentioned above using the detector because the signal-to-noise ratio is below an acceptable limit. As an example, a conventional detector having a signal to noise ratio of forty-to-one prior to any exposure, may be observed to have a signal-to-noise ratio reduced to two- or three-to-one after 10.sup.8 pulses of high energy radiation.
When the signal-to-noise ratio drops to these low levels, it becomes too difficult to resolve spectral features out of the dark current background, and attempts to characterize laser parameters become unreliable. At that point, a new detector is configured, aligned and calibrated to replace the old "worn out" detector. This procedure results in undesirable down time for the system, as well as replacement cost.
As discussed below in the summary, the present invention provides a detector having a frequency conversion coating on its surface, and a laser system including the detector. The coating converts incident ultraviolet light into longer wavelength light. The longer wavelength light impinges upon the detector element without producing the type of dark current background which grows quickly in proportion to total accumulated exposure dose. The lifetime of a detector element having the frequency conversion coating on its surface is thus lengthened dramatically, thus decreasing system downtime and cost.
Quantum frequency converters have been used in the past for changing the energy of incident carriers to another, typically lower, energy. For example, electron bombardment of optical surfaces coated with phosphors, results in the emission of light from the phosphor coating. Prime examples include phosphor television and computer screens incorporating cathode ray tubes.
Quantum converters have also been installed within detection systems employing CCD cameras. See H. S. Albrecht, U. Rebhan, K. Mann and J. Ohlenbusch, Measurement and Evaluation Methods for Beam Characterization of Commercial Excimer Lasers, Proc. of 3rd Int. Workshop on Laser Beam and Optics Characterization (LBOC), Quebec, Canada (July 1996), SPIE Vol. 2870, 367 (1996); K. Mann and A. Hopfmuller, Characterization and Shaping of Excimer Laser Radiation, Proc. of 2nd Workshop on Laser Beam Characterization, S. 347, Berlin, Mai (1994). These quantum converters are incorporated as coated or doped plates for converting ultraviolet light to visible light, the purpose being so that standard visible light refractive imaging systems could be used. The quantum converter used was a Ce or Tb doped glass plate inserted in front of the detector.
The converter plate is displaced from the detector surface and serves as a window to the detector. After conversion to visible light by the plate, the light is imaged by an imaging lens before entering the detector and being detected by a CCD camera. The distance needed for imaging the light after quantum conversion increases the overall packet size of the detector unit. Moreover, procedures for properly aligning the imaging optics are necessary and require extra time and effort.
Until now, only 248 nm radiation from a KrF-laser has been substantially used in photolithography. The 248 nm light has not been observed to produce any substantially increasing dark current background. Now that the use of 193 nm light has brought with it the dark current growth problem, another detection system and technique to alleviate the problem has become necessary.